If I may paraphrase words from "The Charge of the Light Brigade" by Alfred Lord Tennyson -- "Ours is not to reason why, ours is but to do and…" well, maybe not die. More frequently than I like, we get asked to design something that makes no sense to us. Here are some designs that we were not sure should involve a PCB, but did.
A lot of our products get mounted on DIN rails inside panels. Most of the time, the PCB is designed to fit in a housing that is then clipped onto the rail. Figure 1 shows an inversion of this concept with a TS35 DIN rail mounted on a PCB.
Figure 1. A TS35 DIN rail mounted on a PCB.
There is some additional circuitry on the underside of the PCB. The intention is to mount several DIN modules on the rail and connect them to the under-board circuitry. The board fits in a long plastic housing (which can be cut to any length) that protects the bottom of the board, leaving the top exposed. The housing also has feet that allow it to mount on a DIN rail. There are examples of the housing in several of the photos that follow.
In Figure 2 the TS35 rail fits in the slot, which is vertical in this photo. This module doesn't use a housing at all. It actually connects the ground of the PCB to the grounded rail by making contact with the screws that affix the foot to the PCB. For those of you intrigued by the square holes in the screws, this is a great Canadian invention (no slip and the screw stays on the screwdriver) called the Robertson screw.
Figure 2. The TS35 rail fits in the slot.
One of the most time-consuming activities in mounting modules on DIN rails is wiring each module. Since almost every module needs power and ground, the arrangement in Figure 3 is a split TS35 rail with a PC board down one or both of the sides for bussing of the power, ground, and some other signals.
Figure 3. A split TS35 rail with a PC board down one or both of the sides.
The big spring-loaded clip in Figure 4 is meant to hold a coax-type wire in place and simultaneously ground it with the external sheath removed. I believe this is normally supposed to be screwed directly to the metal back-plate in a panel.
Figure 4. A board boasting a spring-loaded clip.
Figure 5 shows a dual socket US power outlet with ground-leakage mounted on a PCB. This is actually completely encased in a plastic housing, which I described in the discussions associated with Figure 1. (You can see a version of this orange housing in Figure 6.)
Figure 5. Dual socket US power outlet with earth-leakage mounted on a PCB.
We actually make a few variations on this power outlet idea. Figure 6 shows one with a built-in circuit breaker. I'm told that the socket is intended to be mounted on a PCB, but the pattern in Figure 7 sure looks unusual. The main thing is that our range of products using this concept has been well received in the marketplace.
Figure 6. Power outlet with a built in circuit breaker.
Figure 7. This pattern sure looks unusual.
In Figure 8 we can see a tiny 5x20mm PCB that acts as a blown fuse indicator. This comprises a resistor and LED. It fits under a 5x20mm fuse. The fuse in this image is shown projecting out the back just to give the size comparison, but in reality it fits right on top of the PCB with its contacts resting on the large pads you can see at either end. The whole assembly clips into a fuse-holding terminal via a hinge. This is one of our best sellers.
Figure 8. A tiny 5x20mm PCB that acts as a blown fuse indicator.
There are times where we want to provide a visual indication on a board, but without any components on the user side (with the exception of connectors). In order to cater for this, all of the components (except the connectors and switch) mount on the solder-side and we use LEDs that allow for reverse mounting like the CMD91-21/TR10 from Chicago Miniature. I guess there must be an equivalent switch, but we haven't found it yet. Figure 9 shows one such matrix of LEDs.
Figure 9. A matrix of reverse-mounting LEDs.
Over the years, we have used some unusual components. In fact, we have even mounted analog meters directly onto PCBs as illustrated in Figure 10.
Figure 10. An analog meter mounted on a PCB.
How about unusual board shapes? Because of thickness limitations, there may be cut-outs to allow pots or other components to be installed in the housing. Also, the need to insert the PCB into the housing without removing the connector pins sometimes necessitates some unusual shapes as shown in Figures 11 and 12.
Figure 11. One variation of an unusually-shaped housing.
Figure 12. Another variation of an unusually-shaped housing.
Our organization doesn't have much in the way of mechanical engineering. This means that it is sometimes simpler for us to make something out of PCB material using the PCB tools rather than subcontracting it out for mechanical design. Figure 13 shows a small piece of PCB material that is used to cover a relay for retention.
Figure 13. A small piece of PCB material used to cover a relay for retention.
Extending this idea, the four PCBs shown Figure 14 are used to make the connector adapter assembly in the lower left. Only one board is used for electrical connections. The other three are used for a combination of isolation, spacing, stiffening, and finger-proofing.
Figure 14. Using PCBs for different purposes, including electrical connections, isolation, spacing, stiffening, and finger-proofing.
Can you make a contribution to this strange brew? (Dang! Now I've got that Cream song running around in my head.) If so, please add your devices to the comments below or email them to Max (firstname.lastname@example.org) for him to post in a future article.